It is known that rare earth oxides are materials of great interest in forming transparent ceramics, e.g. for use as optical components, for example in laser amplifiers, scintillators and ultraviolet (UV) lenses. However, very few material compositions are viable and/or available as large sized transparent ceramic components.
One reason for the main challenges of working with these materials and creating large components is the high melting temperature, which makes sintering to full density particularly difficult. Furthermore, many of these oxides undergo a phase change from cubic to monoclinic with increasing temperature and pressure.
Lu2O3 with Eu is a recently developed material suggested for use in X-ray scintillator screens. However, due to the larger atomic radius of Eu, it is not completely stable in the Lu2O3 lattice and tends undesirably to be exsolvated to the grain boundaries in the ceramic to form secondary phases, which reduce transparency of the resulting ceramic.
Some conventional approaches have included attempting to form translucent ceramics from oxides of europium, lutetium and gadolinium, but none have been able to achieve the desirable near-perfect transparency disclosed herein. For example, the europium doped lutetium gadolinium oxides disclosed in “Effects of Doping Lu2O3 on Phase Transformation and Luminescence” IEEE Trans. Nuc. Sci. 57:1343-47 (2010) to Qin, et al. disclose compositions having poor transparency characteristics, namely having optical transmittance no greater than 75% for a 0.25 mm thick component, which corresponds to a scatter coefficient of approximately 90%/cm. See, e.g. FIG. 4 of Qin, et al.
Applying both temperature and pressure simultaneously is commonly used to fabricate fully dense ceramics in conventional processes. Fully dense ceramics because residual porosity undesirably scatters photons traveling through the medium, decreasing the transparency thereof. However, extreme temperature and pressure is easily triggers cubic to monoclinic phase transformations. Similar to residual pores, secondary phase structures (especially monoclinic phase structures) scatter photons traveling through the medium, and further decrease the transparency thereof.
Therefore, it is exceedingly difficult to achieve a fully transparent ceramic (e.g. over 75% transmittance, less than 10%/cm scatter) according to conventional methods and materials known in the art. Since scatter increases with the length of the path a photon travels through a given medium, it is even more difficult to produce fully transparent ceramics on a macro-scale (e.g. diameter greater than 25 mm, thickness greater than 0.25 mm).
Accordingly, it would be desirable to provide large-scale fully transparent ceramics that do not have the residual porosity and monoclinic structures found in conventional ceramics formed by traditional fabrication methods. Furthermore, it would be advantageous to describe methods of fabricating such ceramics, where the constituent materials do not undergo to cubic to monoclinic phase changes during sintering and/or pressurization and exhibit substantially no residual porosity. These developments would desirably improve the transparency characteristics of ceramic products for use in military, medical, basic research and commercial applications.